What does a "lean" fuel mixture indicate?

A "lean" fuel mixture indicates that there is too much air compared to fuel. This situation occurs when the ratio of air to fuel exceeds the optimal level needed for efficient combustion. In a lean mixture, the combustion process can become incomplete, leading to higher combustion temperatures and the potential for engine knocking or pre-ignition. A lean condition can also result in decreased engine performance and increased emissions, as the fuel does not burn as completely as it would in a balanced mixture.

In contrast, a balanced fuel and air ratio typically results in optimal engine performance, providing the right amount of fuel for the air present, and improving efficiency and power. Excessive combustion pressure is generally associated with incorrect tuning or mechanical issues rather than directly relating to the air-to-fuel ratio.